In a conventional hair dryer, a blower moves air through the dryer onto the hair and an electrical resistance heater is commonly positioned within the dryer to heat the air. In the conventional dryer, however, it has been difficult and expensive to provide a sufficiently large volume of air and to provide adequate capacity for heating such a large volume of air while also assuring that the dryer is safe to operate. That is, if the heater of the conventional dryer were provided with the desired large heating capacity, there would be a significant risk that, if the air intake to the dryer were to be inadvertently blocked or if air supply to the dryer were otherwise cut-off, the large heater could be subjected to catastrophic overheating. As a result, conventional hair dryers intended for the lower price segment of the beauty care market have commonly been provided with somewhat limited air volume and heating capacities.
It had been known that a highly efficient fluid heater could be formed from a body of electrical resistance material having a plurality of fluid passages extending through the body, the heater material having a positive temperature coefficient of resistivity and being adapted to display a sharp, anomalous increase in resistivity when self-heated to a selected temperature by directing electrical current through the body. It had also been known that, if utilized in a hair dryer or the like, such an electrical resistance heater would be self-regulating to eliminate the risk of overheating. That is, when such a heater is electrically energized while a stream of air or other fluid is directed through the heater passages by a blower or the like, heat output from the heater is high and excellent heating efficiency is obtained while the heater remains below the anomaly temperature at which its resistivity sharply increases. The cooling effect of the air moving through the heater passages also tends to maintain the heater in this high efficiency heating mode below its anomaly temperature. On the other hand, if the volume of air directed through the heater passages is reduced or cut-off so that the heater temperature tends to increase to its anomaly temperature, the heater resistivity sharply increases to reduce heater current, to stabilize the heater temperature, and to prevent overheating of the heater material. When operating in this high resistivity mode, the heat output and energy consumption of the heater are sharply reduced. However, when such a multi-passaged heater of positive temperature coefficient of resistivity is considered for use in a hair dryer or the like, it is found that it is difficult to vary the air output temperature of the dryer. It had been found to be particularly difficult to vary the temperature of the stream of air provided by the dryer without also undesirably varying the volume of heated air furnished by the dryer.
It is an object of this invention to provide a novel and improved fluid heater such as a hair dryer; to provide such a hair dryer system which is easily adjusted to regulate the temperature of air or other fluid heated by the system; to provide such a dryer which is highly efficient, which has a high air heating capacity, and which is safe to operate; and to provide such a dryer which is of simple, rugged and inexpensive construction.
Briefly described, the novel and improved hair dryer of this invention comprises a housing which defines an air path through the housing between an air intake and an air outlet. A blower of other comparable means is mounted on the housing for moving a stream of air along the air path and for directing the stream of air from the dryer through the air outlet. The dryer also includes a self-regulating electrical resistance heater formed from a body of ceramic resistance material having a plurality of fluid passages extending through the body. The resistance heater is formed from a material which has a positive temperature coefficient of electrical resistivity and which is adapted to display a sharp, anomalous increase in resistivity when the heater is self-heated to a selected temperature by directing electrical current through the heater material. The heater is mounted within the noted air path for heating air which is moved through the path by the blower means. In accordance with this invention, means are provided for adjusting the heat-transfer relationship between the heater body and the stream of air moving through the noted air path, thereby to vary the degree of heat-transfer between the body and the air to regulate the temperature of the air furnished by the dryer.
In one preferred construction, the heater body has a disc configuration and has its fluid passages extending in side-by-side relation between the broad flat surfaces of the heater disc. In this construction, the adjusting means preferably comprises means mounting the heater body for rotational movement between a first position having the heater passages extending parallel to the longitudinal axis of the air path so that substantially all air moving along the path is directed through the heater passages and a second position having the heater passages disposed oblique or perpendicular to the air path axis so that a substantial part of the air moving in the path passes around the heater without passing through the heater body passages. Air heated by being passed through the heater passages is then commingled with air passed around the heater to determine the temperature of the air stream furnished by the dryer. In this arrangement, when the heater body is moved from its first position toward its second position, the temperature of the air directed from the air outlet of the dryer is adjusted from an initial high air temperature to a relatively much lower air temperature.
In another preferred construction, the heater body has a disc-like ring configuration having a central opening of substantial size and having the heater body passages extending between opposite broad, flat disc surfaces of the heater ring. In this construction, the adjusting means preferably comprises a baffle disc rotatably mounted within the opening in the heater ring for movement between a first position perpendicular to the axis of the air path where the baffle blocks the central opening in the heater ring and directs substantially all of the air moving in the path through the heater passages and a second position extending parallel to the path axis where the baffle permits a substantial portion of the air moving in the air path to pass through the central opening in the heater ring without passing through the other passages of the heater body. In this arrangement, when the baffle is moved from its first position toward its second position, the temperature of the air directed from the dryer outlet is also adjusted from a high initial air temperature to a relatively much lower air temperature.